Is detachment and rearguard the same thing?
In TAO2, units could generate strong points, which would need to be attacked like a normal unit before being removed from play.
In Korsun Pocket, "Detachments" have replaced the TAO2 version of strongpoints - they are different in that they will only slow down an attacking force, they only last one turn, and they negate the positive odds shift when an enemy unit is attacking from a hex with a Detachment in it.
I have not seen "rearguard" terminology used within the manual, but it is a good descriptor of what a Detachment is - a small force (or all the guys who drew the short straws!) left behind to hinder the advance of the enemy.
Cheers, Chris